Royal Astronomical Society. 52S 



rested against short iron pillars fixed in the breech-piece of the tele- 

 scope, the firm hold produced by friction of the edge of the speculum 

 on the pillars, combined with the varying elasticity of the triangular 

 lever-supports in different inclinations of the telescope, caused 

 distortion of the mirror ; and that when, to avoid this friction, the 

 mirror was suspended (as regarded its edge-bearing), either by a 

 semicircular hoop or by a chain, a small difference in the edgewise 

 pressure, depending on a difference of inclination of the telescope, 

 threw some of the points of the supporting levers out of bearing, and 

 distortion of the mirror was produced. This latter fault arose from 

 the circumstance that the mirror could not slip freely over the points 

 of the supporting levers. To remedy this, the following arrange- 

 ment is now made. Each of the plates resting on the 27 supporting 

 points, instead of being partially attached to the mirror by a layer of 

 felt and pitch, is completely separated from it, to the distance of 

 about 1^ inch, and the speculum rests upon each plate by three 

 turned brass balls at the three angles of the plate ; so that the whole 

 surface of the speculum is now supported by eighty-one brass balls. 

 Each of these balls has a fine wire passing through a small hole in 

 the plate, and kept in tension by a weak spring on the opposite side ; 

 this prevents the ball from rolling away when the mirror is detached, 

 but allows entire freedom of motion to the ball, to the extent of about 

 one inch in any direction. Lord Ilosse has reason to think that this 

 construction is perfectly successful for its object. He has already 

 found that the speculum may be moved laterally half an inch without 

 the smallest discoverable distortion. Before the balls were used, 

 when the speculum was moved laterally -a^th of an inch, vision was 

 destroyed. 



Lord Rosse thinks it, however, desirable to aiTange the edgewise 

 support, so that as little as possible may be trusted to the motion of 

 the balls. The lower semicircle of the edge is now to be gi'asped by 

 a strong iron hoop, very neatly fitted to it ; and the upper semicircle 

 is to have a thin hoop furnished with a drawing-screw or contracting 

 screw, merely for the purpose of bringing the strong hoop constantly 

 into fair contact with the edge of the mirror. And the ends of the 

 strong hoop (which are at the extremity of a horizontal diameter of 

 the mirror) are to be supported by rods, attached to a horizontal bar 

 which rests on the two upper pillars of the breech-piece, with the 

 utmost freedom of motion ; so that the mirror will be supported 

 edgewise by jimbals of the most perfect construction. Lord Rosse 

 hopes that, with this arrangement, in combination with the support 

 on the balls already described, the mounting of the mirror will be 

 sensibly perfect. 



Lord Rosse had also communicated to the Astronomer Royal some 

 remarks upon the process of grinding large mirrors. With mir- 

 rors of 3 feet aperture there is not the smallest difficulty. In the 

 mirrors ground and polished by his apparatus, there is no appreciable 

 difference of focal length of the central part and the annulus next 

 the edge ; and this result is obtained uniformly. Still, with the 

 6-feet mirrors there is great difficulty. In all cases, a figure is ob- 



